1. Field of the Invention
The present invention is in the field of power converters. The present invention is further in the field of semiconductor switching power converters. The present invention further relates to the field of integrated hysteretic control methods for switching power converters and circuits. The implementation is not limited to a specific technology, and applies to either the invention as an individual component or to inclusion of the present invention within larger systems which may be combined into larger integrated circuits.
2. Brief Description of Related Art
Modern electronic applications require power management devices that supply power to integrated circuits or more generally to complex loads. In general, power switching converters are becoming more and more important for their compact size, cost and efficiency. The switching power converters comprise isolated and non isolated topologies. The galvanic isolation is generally provided by the utilization of transformers. The subject invention refers to isolated and non isolated power converters.
Modern switching power converters are in general divided in step down power converters also commonly known as “buck converters” and step up power converters commonly known as “boost converters”. This definition stems from the ability of the converter to generate regulated output voltages that are lower or higher than the input voltage regardless of the load applied.
Boost converters can be implemented by storing and releasing energy in a passive component and more precisely in a capacitor or in an inductor. In particular the case of capacitive charging is also known as charge pump converter while, when the inductor is used, the converter is generally known as inductive boost converter.
Inductive boost converters are very important to generate well regulated voltage rails at voltages higher than the input voltage available. Typically, this is obtained by first charging the inductor with energy by applying a current through it and thereafter switching off a terminal of the inductor so as to discharge the current into a load at higher voltage. The most known and used prior art for a switching non isolated inductive boost converter topology is shown in FIG. 1.
The modes of operation of inductive switching power converters are mainly two. The first is the Continuous Conduction Mode (CCM) characterized by the fact that, at steady state, the inductor current increases and decreases with the switching frequency and duty cycle but it is never kept at zero during the duty cycle. CCM generally occurs when the load current is high enough to require a positive inductor current and therefore a constant flow of energy from the input to the output. If and when the inductor current crosses a zero value the converter can be kept in CCM by allowing the inductor current to become negative and therefore discharging the output capacitor.
The second mode of operation is the Discontinuous Conduction Mode (DCM) characterized by the fact that when the inductor current reaches zero value, it is kept at zero for part of the period. This second mode is generally entered to when the load current is small. If the load current is not very large the output capacitor can provide enough energy to the load for part of the switching period so that during that time interval the inductor energy can be null. Typically, in DCM the output voltage ripple is more pronounced since the energy is stored also in the output capacitor so as to allow lower switching frequency.
Fast control of boost converters is difficult to obtain in CCM because there is always an intrinsic delay in providing energy to the load since the inductor has to be first charged with current flowing in it. If the load suddenly changes from a low current to a high current load, the boost converter circuit has to spend some time to charge the inductor first and during this time no current/energy is supplied to the load. This phenomenon is not present in buck converters where by applying current to the inductor, the same current is flowing in the load as well.
The small signal analysis of the boost circuit in CCM points out to the presence of a right half plane zero (RHPZ). This is the effect that an increase of load current causes an apparently counter-intuitive decrease of the current in the diode due to an increase of duty cycle. This RHPZ can complicate the stability of the loop and generally is dealt with by rolling off the loop gain of the switching voltage regulator at relatively low frequency, making the overall response of the boost converter quite slow.
Generally the boost converters are controlled with PID (proportional-integral-derivative) type of control method. In particular current mode controls are quite common because they include two nested loops: one for the control of the output voltage and one for the control of the output current. However, as mentioned, these types of control methods do not present high bandwidth and require the adoption of large output capacitors to obtain acceptable load transient responses.
High frequency switching power converters are increasingly more popular due to the advantage of using low value inductors and capacitors reducing significantly the cost and board space of the power management section. Buck converters can successfully be operated at high frequency by using hysteretic and pseudo-hysteretic approaches. Generally the control loop of pseudo-hysteretic converters is relatively simple and the output voltage is summed to a ramp signal to generate a synthetic ripple. A prior art example of pseudo hysteretic switching buck converter is provided in Rincon-Mora (U.S. Pat. No. 6,369,555).
Generally this synthetic ripple signal is fed to a fast comparator that determines the charge and discharge timing of the inductor. For buck converters the implementation of a pseudo hysteretic control is relatively simple because the output stage of the buck, along with the inductor and the output capacitor, forms the integrating section of the converter that can be seen as a delta sigma converter. As mentioned, the buck converter charges the inductor while supplying current to the load.
The intrinsic delay of the boost architecture, deriving from the fact that the boost does not supply current to the load while charging the inductor, makes the implementation of an hysteretic approach much more difficult to obtain. However there are prior art attempts to achieve a hysteretic control of boost converter like what has been proposed by Mei et al (U.S. Pat. No. 7,626,370). In this case a ripple signal is generated by adding a resistor in series to the power switch device and the output filter capacitor of the boost power converter. However the proposed architecture presents two major drawbacks: a higher power dissipation because the resistor is placed directly on the power path, and a high output ripple because the resistor is in series to the output capacitor.
It is therefore a purpose of the present invention to describe a novel structure of a switching boost converter with synthetic ripple generation that can operate at high switching frequency with pseudo-hysteretic control and synthetic ripple generation, operating with high efficiency both in CCM and DCM depending on the load conditions.